1. Technical Field
Improved nutating pumps are disclosed with piston designs that provide output flow in both the first and second parts of the piston rotation/reciprocation cycle thereby providing about, half the normal flow rate during the first part of the cycle as a conventional piston but also about that same flow rate during the second part of the cycle in contrast to prior art nutating pumps where there is no flow rate for second part or intake portion of the cycle. The result is smoother, reduced pulsation flow and an overall cycle dispense amount about equal to a conventional nutating pump but with less pulsations and splashing. The nutating pumps have numerous applications where accuracy and speed are important.
2. Description of the Related Art
Nutating pumps are pumps having a piston that both rotates about its axis liner and contemporaneously slides axially and reciprocally within a line or casing. The combined 360° rotation and reciprocating axial movement of the piston produces a sinusoidal dispense profile that is illustrated in FIG. 1A. In FIG. 1A, the sinusoidal profile is graphically illustrated. The line 1 graphically illustrates the flow rate at varying points during one revolution of the piston. The portion of the curve 1 above the horizontal line 2 representing a zero flow rate represents the output while the portion of the curve 1 disposed below the line 2 represents the intake or “fill.” Both the pump output and pump intake flow rates reach both maximum and minimum levels and therefore there is no linear correlation between piston rotation and either pump output or pump intake.
The colorant dispensers disclosed in U.S. Pat. Nos. 6,398,513 and 6,540,486 (Amsler '513 and Amsler '486) utilize a nutating pump and a computer control system to control the pump. Prior to the system disclosed by Amsler et al., existing nutating pumps were operated by rotating the piston through a full 360° rotation and corresponding axial travel of the piston. Such piston operation results in a specific amount of fluid pumped by the nutating pump with each revolution of the piston. Accordingly, the amount of fluid pumped for any given nutating pump is limited to multiples of the specific volume. If a smaller volume of fluid is desired, then a smaller sized nutating pump is used or manual calibration adjustments are made to the pump.
For example, in the art of mixing paint, paint colorants can be dispensed in amounts as little as 1/256th of a fluid ounce. As a result, existing nutating pumps for paint colorants can be very small. With such small dispense amount capabilities, the motor of such a small pump would have had to run at excessive speeds to dispense larger volumes of colorant (multiple full revolutions) in an appropriate time period.
In contrast, larger pumps may be used to minimize the motor speed. When small dispense amounts are needed, a partial revolution dispense for such a larger capacity nutating pump would be advantageous. However, using a partial revolution to accurately dispense fluid is difficult due to the non-linear output of the nutating pump dispense profile vs. angle of rotation as shown in FIG. 1A.
To address this problem, the disclosures of Amsler '513 and '486 divide a single revolution of the pump piston into a plurality of steps that can range from several steps to four hundred steps or more. Controllers and algorithms are used with a sensor to monitor the angular position of the piston, and using this position, calculate the number of steps required to achieve the desired output. Various other improvements and methods of operation are disclosed in Amsler '513 and '486.
The sinusoidal profile illustrated in FIG. 1A is based upon a pump operating at a constant motor speed. While operating the pump at a constant motor speed has its benefits in terms of simplicity of controller design and pump operation, the use of a constant motor speed also has inherent disadvantages, some of which are addressed in U.S. Pat. No. 6,749,402 (Hogan et al.).
Specifically, in certain applications, the maximum output flow rate illustrated on the left side of FIG. 1A can be disadvantageous because the output fluid may splash or splatter as it is being pumped into the output receptacle at the higher flow rates. For example, in paint or cosmetics dispensing applications, any splashing of the colorant as it is being pumped into the output container results in an inaccurate amount of colorant being deposited in the container but also colorant being splashed on the colorant machine which requires labor intensive clean-up and maintenance. Obviously, this splashing problem will adversely affect any nutating pump application where precise amounts of output fluid are being delivered to an output receptacle that is either full or partially full of liquid or small output receiving receptacles.
For example, the operation of a conventional nutating pump having the profile of FIG. 1A results in pulsed output flow as shown in FIGS. 1B and 1C. The pulsed flow shown at the left in FIGS. 1B and 1C, at speeds of 800 and 600 rpm respectively, results in pulsations 3 and 4 which are a cause of unwanted splashing. FIGS. 1B and 1C are renderings of actual digital photographs of an actual nutating pump in operation. While reducing the motor speed from 800 to 600 rpm results in a smaller pulse 4, the reduction in pulse size is minimal and the benefits are offset by the slower operation. To avoid splashing altogether, the motor speed would have to be reduced substantially more than 20% thereby making the choice of a nutating pump less attractive despite its high accuracy. A further disadvantage to the pulsed flow shown in FIG. 1A is an accompanying pressure spike that cause an increase in motor torque.
In addition to the splashing problem of FIG. 1A, the large pressure drop that occurs within the pump as the piston rotates from the point where the dispense rate is at a maximum to the point where the intake rate is at a maximum (i.e. the peak of the curve shown at the left of FIG. 1A to the valley of the curve shown towards the right of FIG. 1A) can result in motor stalling for those systems where the motor is operated at a constant speed. As a result, motor stalling will result in an inconsistent or non-constant motor speed, there by affecting the sinusoidal dispense rate profile illustrated in FIG. 1A, and consequently, would affect any control system or control method based upon a preprogrammed sinusoidal dispense profile. The stalling problem will occur on the intake side of FIG. 1A as well as the pump goes from the maximum intake flow rate to the maximum dispense flow rate.
The splashing and stalling problems addressed by Hogan et al. are illustrated partly in FIG. 2 which shows a modified dispense profile 1 a where the motor speed is varied during the pump cycle to flatten the curve 1 of FIG. 1A. The variance in motor speed results in a reduction of the peak output flow rate while maintaining a suitable average flow rate by (i) increasing the flow rates at the beginning and the end of the dispense portion of the cycle, (ii) reducing the peak dispense flow rate, (iii) increasing the duration of the dispense portion of the cycle and (iv) reducing the duration of the intake or fill portion of the cycle. This is accomplished using a computer algorithm that controls the speed of the motor during the cycle thereby increasing or decreasing the motor speed as necessary to achieve a dispense curve like that shown in FIG. 2.
However, the nutating pump design of Hogan et al. as shown in FIG. 2, while reducing splashing, still results in a start/stop dispense profile and therefore the dispense is not a pulsation-free or completely smooth flow. Despite the decrease in peak dispense rate, the abrupt increase in dispense rate shown at the left of FIG. 2 and the abrupt drop off in flow rate shown at the center of FIG. 2 still provides for the possibility of some splashing. Further, the abrupt starting and stopping of dispensing followed by a significant lag time during the fill portion of the cycle still presents the problems of significant pressure spikes and bulges and gaps in the fluid stream exiting the dispense nozzle. Any decrease in the slope of the portions of the curves shown at 1a, 1c would require in increase in the cycle time as would any decrease in the maximum fill rate. Thus, the only modifications that can be made to the cycle shown in FIG. 2 to reduce the abruptness of the start and finish of the dispensing portion of the cycle would result in increasing the cycle time and any reduction in the maximum fill rate to reduce pressure spiking and motor stalling problems would also result in an increase in the cycle time.
Accordingly, there is a need for approved nutating pump with an improved control system and/or a method of control thereof where by the pump motor is controlled so as to reduce the likelihood of splashing and “pulsing” during dispense without compromising pump speed and accuracy.